Your title is about "longer clips" but your message is about "playback being really choppy"
Those are two DIFFERENT issues... as has been pointed out to you
There is a BUG in CS6 concerning spanned clips... as you say in message #2
Choppy playback is more a function of hardware configuration... which is total hardware, not just CPU and memory... and which includes the number and speed of hard drives, and where you have your files to allow the best flow of data
The absolute minimum for EFFECTIVE video editing is one 7200rpm hard drive for operating system and all software, and a second, separate 7200rpm hard drive for all project and video files
An eSata or USB3 external drive may work